This invention is related to the invention described and claimed in our Klebe, et al copending application Ser. No. 837,262 filed Sept. 27, 1977, now U.S. Pat. No. 4,156,039. The entire disclosure of the copending application is hereby incorporated by reference and relied upon.
The use of sodium percarbonate as a bleaching agent alone or in washing agents is of increasing interest since the washing process is increasingly carried out at lower temperatures down to as low as room temperature.
In contrast to sodium perborate tetrahydrate (NaBO.sub.2.H.sub.2 O.sub.2.3H.sub.2 O) (theoretical active oxygen=10.4%) sodium percarbonate has the advantage of greater solubility at 20.degree. C. and a greater active oxygen content (theoretical active oxygen=15.3%). However, it has the severe disadvantage that it has very little storage stability in a non dry atmosphere. Thus, the slight amounts of moisture which are present already in the atmosphere and/or in the washing agent are sufficient to bring about the decomposition of the percarbonate.
Especially in warmer zones, where a higher temperature prevails, the decomposition once initiated progresses rapidly.
This problem was known early and therefore for a long time there have been efforts to increase the storage stability of sodium percarbonate in various ways.
Thus, there has been attempted the obtaining of increased stability with the help of aerosols such as silica aerosols (German Pat. No. 870,092).
Also, by the addition of benzoic acid the stability should be increased (German OS No. 1,767,796).
The additives were added either during the production of the percarbonate or in connection with the production of the finished product.
It has already been tried to obtain a stable product by using stabilizers and complex formers which eliminate the impurities of the added soda or the impurities which appear during the process. (German OS No. 2,234,135).
Another idea which has been tried repeatedly is to protect the percarbonate particles by encasing with a protective layer before the decomposition influences take effect. Thus, according to French Pat. No. 893,115 there have been used natural or synthetic resins, or according to German OS No. 2,511,143 polyethylene glycol, or there have been used copolymers in which one of the monomers was vinyl chloride or vinylidene chloride (German OS No. 2,402,392 and 2,403,393).
There has also been proposed encasing with a sodium silicate solution (British Pat. No. 174,891) or a mixture of alkali or alkaline earth silicate and silicoflouride (German OS No. 2,511,143) or an aqueous sol of silicates (Belgian Pat. No. 820,741).
However, it is indicated that the stability is only retained if the storage is in dry air at 20.degree. C.
In any case, however, whatever the reason there was always present so much moisture that the decomposition of the percarbonate began, the storage temperature increased due to the heat of decomposition which accelerated the further decomposition of the percarbonate as did the water obtained from the hydrogen peroxide decomposition.
Thus, there occur active oxygen losses of 40% and more within 15 days if the stability test is carried out at 40.degree. C. and 80% relative air humidity (German OS No. 2,511,143).
The object of the invention therefore is the production of a percarbonate relatively stable in a moist and warm environment, whose loss of activity in the same type of environment is still comparatively small and which retains its good speed of solubility after the stabilization.
In Klebe, et al U.S. application Ser. No. 837,262 filed Sept. 27, 1977 and related published German application P26 51 442.1-41 there is described stabilized sodium percarbonate particles comprising a sodium percarbonate nucleus encased in a layer of a mixture of dehydrated sodium perborate and sodium silicate prepared by applying to the sodium percarbonate an aqueous sodium silicate solution and then applying to the wet sodium percarbonate particles dehydrated sodium perborate. The dehydrated sodium perborate and sodium silicate can be mixed with each other.